Primers, or detonators as they are alternatively known, are the means used to detonate an explosive charge. Those devices may be detonated by electrical or mechanical means. They are comprised of a primary explosive which may be initiated by stab, friction or impact, an intermediate explosive composition that is set off by the primer composition, and a base charge of a secondary explosive such as RDX or HMX to provide the desired explosive output. These primers are ordinarily coupled with booster charges and a propellant to form an explodable ignition chain.
Primer/detonator compositions are employed in a variety of applications, including weapon systems, razing buildings, and in creating excavations. Primer/detonators are also used to initiate the explosive chain in automobile air bag systems. Explosive charges are uniquely suited for these systems because the explosive reaction instantaneously generates a large volume of gas that fills the air bag. It is essential that the air bag be filled within that instant between collision and the occupant's impact with the dashboard.
As the primer/detonator functions to initiate the explosive reaction, the primer must be capable of being set-off by the energy provided by the crash. This is acheived by using mechanical or chemical sensitizers in the primer composition used in the primer. Since the energy provided is of a low order, mechanical sensitizers are unable to set-off the primer composition. Chemical sensitizers are capable of great sensitivity so that the primer compositions are set off at low impact energy. The sensitizer must also be capable of not being desensitized at temperatures higher than 95.degree. C. Therefore sensitizers like tetrazene are not useful, which decompose at temperatures as low as 80.degree. C.
Airbag systems are ordinarily housed in a metallic structure. Copper is a favored material for housing the primer as it is lightweight and capable of withstanding the explosive forces generated during detonation.